The show is fond of dropping in historical figures these days. Shakespeare, Dickens, Van Gogh, Queen Victoria, Louis XV, Nixon and Churchill have all popped up since the series was rebooted six years ago. It’s a device which allows the writers to give viewers a playful history lesson, while offering extraterrestrial explanations for past events. Inform, educate and entertain… Lord Reith would approve, although he’d probably be baffled by this plot.

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The script contained nods to several films: Nazi motorbikes were stolen like The Great Escape, Kingston purred a Mrs Robinson-ish “Hello, Benjamin”, some of the CGI sci-fi tricks recalled Men in Black and The Terminator. “Whopremo” Steven Moffat has compared his complex plotting to Inception, and he does tend towards the tricksy. This was jam-packed full of ideas, twists, turns and wibbly-wobbly time-bending stuff. Giddily thrilling entertainment, albeit rather exhausting. I don’t know how the Doctor does it at his age (a sprightly 909 at last count) but I wouldn’t mind being him when I grow up, either.

Meanwhile, the Telegraph's Doctor Who expert Gavin Fuller wonders if it was a wasted opportunity:

Packing plenty into an episode is all very well, but there is a fine line to be walked between over-egging the style and allowing plots to breathe, and this episode wasn’t totally successful in that count. Although it was enjoyable enough, and we certainly learnt a lot of the back story of Amy, Rory and River, I was left feeling that much more could have been done with the setting. Indeed, much of the story could have been set anywhere and any time, which is a waste of using such a resonant historical period as the backdrop.

For all that, to me Let's Kill Hitler was far more successful as a season opener than A Good Man Goes To War was as a finale. Here was an energetic, timey-wimey tour de force with with gags and flourishes like the car and the crop circles that still maintained a strong sense of what it was about. Most fabulously of all, it was all about Doctor Song. ... If you could keep up, we were given a lot more answers than we might have dared to expect. Yes she did have regenerative powers, but in saving the Doctor she also sealed her fate to that ultimate 'death' in the Library. We learn where she got the Tardis diary. But we still have to deal with the mystery of who she is to the Doctor. Perhaps most brilliantly of all, we solve the continuity niggle of Alex Kingston's reverse ageing: "I might take the age down a little, just gradually, just to freak people out."

Shape-shifting robots and miniaturisation rays in Doctor Who are to be encouraged. But is there an argument, somewhere, that having River/Melody perceived by the people in the Tessalator as a worse war criminal than Hitler maybe, possibly, a little bit dodgy?

Doctor Who... the usual ball of nerdy confusion as the Doctor and his time-travelling chums hurtled into 1939 Berlin and locked Hitler in the cupboard. Hee hee. A few amusing one-liners, superior special effects... and guest star Alex Kingston’s spirited portrayal of Amy Pond’s demonic daughter Melody. But what was it all about? Don’t ask me. Roll on The Silence.

Given the dark and depressing tone of A Good Man Goes To War, this episode lifted the mood and made things feel a lot lighter, possibly to create a balance. There were some great slapstick moments when River and the Doctor are trying to second-guess one another. River ends up pointing a banana in the Doctor’s face rather than a gun. Also, the Rory death count has begun and it is only a matter of time before it happens.

Moffat’s script takes pleasure in wrongfooting you from the start, packing the episode with never-saw-that-coming moments and ingenious reveals. When that red sports car skids up to the TARDIS before the credits, it seems logical that River Song should step out, but no, it’s Mel… Who later turns out to be River Song anyway. Then there’s the Nazi officer-impersonating robot that turns out to be a vessel packed with hundreds of tiny people – very Men In Black – who travel around time and space dishing out justice to war criminals. An ingenious idea, brilliantly delivered – the morphing effects are Hollywood good.

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Indeed, this has to rank among the cleverest Who episodes Moffat has ever written. After the intensity of “A Good Man Goes To War”, we needed something lighter – which “Let’s Kill Hitler” is – yet Moffat manages to mix the gags and silliness with genuine emotion, and some important additions to the season’s arc plot. Like the “birth” of River Song.

The omission of sorts from the episode was actually Adolf Hitler. He was basically the MacGuffin here, in much the same way that the cybermen were teased in A Good Man Goes To War, and then blown up inside five minutes. In the case of Hitler, he had a few (good) jokes made at his expense, and then got locked in the cupboard. And left there. Let’s Kill Hitler, instead, was far more interested in complicating the relationship between its central characters, which it did terrifically well. Coupled with some of the snappiest dialogue of the show this series, it packed plenty into its near-fifty minute running time. It offered a stark reminder, too, that “the Doctor lies”. As if we didn’t know.

Doctor Who got off to a marvelously energetic, funny, clever, noble mid-season start on Saturday night with the episode titled “Let’s Kill Hitler.” Resolving the cliffhanger of the seventh episode by, with devilish perversity, raising more questions and introducing more plot lines — shaggy-dog story-telling being part of the series’ enduring charm — Doctor Who jumped across time and space in Steven Moffat’s witty script. ... As usual, Smith, Gillan, and Darvill played their roles with dash, while the show grounds them in some authentic emotion. As much fun as it was to see the morphing of River Song, it does leave Amy and Rory childless, doesn’t it? While the Teselecta got under the skin of various people, the series itself gets under the skin of its main characters, and its audience, in a unique manner that continues to play out.